Known mobile communications systems comprise a base station ("the base") that communicates with one or more mobile end systems ("the mobile") over a wireless communications link. There are typically a number of bases logically interconnected by a local area network serviced by a serving terminal in a central office. The serving terminal acts as a router and the base to central office. Communication occurs via a land line or satellite link. As in the phone system, the central office performs a switching and directing function, for further transmission of voice and/or data to a destination communication device. The wireless communications link between the mobile and the base typically operates over a radio frequency ("RF") channel in an appropriate communication band using any one of a number of known modulation techniques.
Under conventional roaming techniques, as an established wireless communications link degrades through movement of the mobile about a geographic area, either the mobile or the base makes a decision that the mobile will roam, i.e., attempt to establish communication over another channel in anticipation of an improved communication link. The mobile then attempts to establish another communication link to replace the degraded link. The mobile attempts to establish communication on a new RF channel either with the same base or with a new base. Conventionally, the decision to roam is based on a measurable degradation of the RF signal between the base and the mobile. Decisions may be based on one or more criteria including, but not limited to, signal strength or noise figure of the signal as received by either the base or the mobile. In voice and data systems, a decision to roam may also be based on data transmission criteria such as bit error rate, transmission success rate, or channel utilization. As one of ordinary skill in the art can appreciate, the conventional roaming criteria, therefore, are typically based upon the quality or status of the RF channel between the mobile and the base and the ability to transmit information over the physical medium.
There is a failure mode in a communications link, however, that can occur in the presence of a high quality physical communications link between the base and the mobile. The communications link between the base and a serving terminal or central office can fail, and like links in a chain, when one link is broken, so that the entire chain and communication degrades or ceases altogether. In this failure mode, the mobile does not attempt to find a new and operable channel because the RF communication between the mobile and the base is functioning properly and the thresholds of the roaming criteria are not surpassed even though information transfer cannot be completed. Consequently, neither the mobile nor the base determines that roaming is necessary or advantageous. The mobile remains in a static state of non-communication in contravention of the objective of the overall communications system.
There is a need, therefore, for an improved method of maintaining a communication link in a wireless communication system to ensure information transfer during a communications session with a mobile in a wireless communications system.